1. Field of the Invention
The subject matter of this invention is an induction furnace crucible and hearth for the incineration and vitrification of organic matter, the vitrification of radioactive and non-radioactive waste, the vitrification of hazardous waste and the fusing of refractory bodies.
2. Description of the Related Art
The structure of said furnaces essentially comprises a hearth in refractory concrete including cooling water circuits on which a side wall is arranged called the crucible, surrounded by an inductive coil in which an electric current circulates at frequencies higher than 100 kHz which is the source of the power produced inside the crucible to melt the matter therein. These furnaces are chiefly used for the incineration and vitrification of organic matter, the vitrification of radioactive or non-radioactive waste and the fusion of refractory bodies. The industries likely to have recourse thereto are waste treatment industries including nuclear and hazardous waste treatment, and the glass industry.
The side wall of the crucible is normally in metallic material permeable to magnetic fields. It contains a cooling circuit so that, firstly, the wall can resist the very high temperatures reached to melt refractory materials such as glass and, secondly, it can compensate for the electric power dissipated by the joule effect within the structure. Said crucible is called a “cold crucible”. In addition it is normally divided into vertical segments, joined by their transverse faces by interposing an electric insulation material to limit currents induced in the wall which would cause heat losses and electromagnetic coupling between the inductor and the content of the crucible. The vertical segments are arranged similar to barrel staves. The cooling circuit usually consists of vertical channels bored in each of the segments.
The segments of the side wall of the crucible must be held together. One first means consists of surrounding the crucible with circular banding in cement or glass fabric impregnated with elastomer or epoxy resin. Another means, offering greater cohesion, consists of welding the segments to one another on a circular flange above the inductor where the intensity to magnetic fields is lower. A last type of assembly which is preferred for the invention consists of assembling the vertical segments and forming the ferrule by screws on a circular flange above the inductor. To facilitate assembly, the segments are provided with assembly lugs on the part mounted outside the ferrule.
The hearth supporting the ferrule is made up of metallic boxes through which a cooling circuit passes, the boxes being placed in refractory concrete, or consists of metallic tubing of various section (round, square, rectangular etc.) mounted in parallel or in chevrons and placed in refractory concrete. The boxes or tubes are separated from one another by a width of refractory concrete. One of the faces is positioned so as to lie perfectly opposite the content in fusion within the furnace. Similar to the tubing, the boxes may be of diverse shape: rectangular, triangular etc.
Known crucibles and hearths suffer from deficiencies which can be detailed as follows. For application to the combustion-vitrification of organic matter over molten glass baths, or the fusion of refractory bodies in an induction furnace, the frequencies and heats required are much higher than for other applications. Risks of electric short circuits may occur between the metallic elements forming the cold crucible (segments, flanges), forming the hearth supporting the crucible (cooled metallic boxes) and between the parts of the crucible and the hearth. These short circuits occur even when the electric insulation placed between the crucible segments and the hearth cooling boxes is of large width.
Without being exhaustive, these electric short circuits between the crucible segments and the hearth boxes are possible through the presence of carbon deposited on the inner walls during the combustion of organic matter, or through the formation of pools of sulphates on the surface of the glass baths entering into contact with different segments and the electric insulations in the inter-segments, or for example through the release of a large quantity of water at the time of fusion of refractory oxides. These short circuits cause irremediable damage to the electric insulations positioned between the parts forming the crucible, to the refractory concrete placed between the hearth cooling boxes, or can even pierce the metallic elements of the hearth and crucible. These electric short circuits are also harmful to efficient use of induction energy.
In the aforesaid applications, corrosive atmospheres at high temperatures are produced, which damage the metallic parts of the furnace consisting of the crucible and hearth, or require the same to be built in materials having high electric resistivity, considerably increasing electric losses.
Irrespective of the shape of the crucible segments (parallelepiped, T-shaped, triangular.) and of the hearth, the sharp edges of these adjacent metallic parts are the source of substantial electric arcs (electric spiking effect). Operating schedules are the chief contributors towards this onset of electric arcs, schedules demanding frequencies greater than 100 kHz for glass applications and waste treatment over molten glass baths. These electric arcs are energetic and harmful to the resistance of the electrical insulations of the crucible and hearth concrete. It is specified that if the crucible segments were round or ovoid, this would eliminate spiking effects but to the detriment of the imperviousness of the furnace ferrule by reducing too far the thickness of the electric insulation between the segments, which would lead to problems of matter and gas leakages as soon as the insulating material shows slight deterioration.
It is to overcome these disadvantages that a new type of crucible and hearth for an induction furnace is put forward as the invention.